For some time now, polyurethane foams with a variety of different physical properties have been commercially produced by the isocyanate polyaddition process from compounds containing several active hydrogen atoms. Generally, such foams are produced from compounds containing hydroxyl and/or carboxyl groups, and polyisocyanates, in the presence of water and/or organic blowing agents and, optionally, catalysts, emulsifiers and other additives (Angew. Chem. A, 59 (1948), page 257). It is possible by suitably selecting the components to obtain both elastic and rigid foams and any intermediate between these two extremes.
Polyurethane foams are preferably obtained by mixing liquid components. Thus, in general, the starting materials to be reacted with one another are either simultaneously mixed, or an NCO-group-containing prepolymer is initially prepared and subsequently foamed.
Tertiary amines have proved to be effective catalysts in the production of polyurethane foams particularly because they are able to accelerate both the reaction between hydroxyl groups and/or carboxyl groups and the NCO-groups, and the reaction between water and the isocyanate groups, with the result that, in the one-shot process, the concurrent reactions may be adapted to one another.
In addition, the foaming process is accompanied by additional crosslinking reactions which result in the formation of allophanate, biuret and cyanurate structures. In view of the complexity of the reactions, it is necessary on the one hand to guarantee their synchronous completion by selecting suitable catalysts and, on the other hand, to ensure that the catalyst is not prematurely fixed by incorporation in the foam or does not subsequently interfere with hydrolytic degradation of the finished foam. In addition, the odor of numerous, commonly used tertiary amines is unfavorable for use in foams.